Linear guides are an essential component when it comes to building 3D printers, CNC machines, and other machines that need some kind of linear motion. Unfortunately, the high quality linear guide rails are often too expensive for many hobby projects. Cheaper ones are available from China, but they can come with quality issues that limit their effectiveness. Luckily many of these issues can be addressed in order to improve the performance.

A MGN12 linear rail with carriage on top of a cutting mat.
A typical MGN12 (12mm wide) linear rail with carriage.

The Chinese linear guides work well for the most part, but they can come with some problems.

For example, the carriage can be ‘sticky’ and not travel smoothly, or the carriage can get stuck altogether at some points along the rail. In this article I will give some methods to fix this.

These techniques will not solve all issues, but work in my experience often enough to be worth trying.

Before explaining how to fix the cheap linear guides, I will give some more information on the different quality linear guides that are available on the market.

Buying linear guide rails

From what I can tell there are several quality classes to choose from when buying linear guide rails. They can be divided in roughly three categories, all with different prices, tolerances and build quality:

Cheap Chinese linear guides

These are the linear guides that can be found for very low prices on AliExpress or eBay. Out of all the available linear guides on the market, these are the cheapest. Their low price is reflected in the build quality. Inferior materials (compared to the more expensive ones) are used, and the machining quality is also usually not that great.

These guides tend to be a bit hit & miss. Some of the units work well, others have issues. Because of their low price it is possible to buy some extra ones and handpick the rails & carriages that work best. The guides with minor issues can often be fixed. Helping you do so is the goal of this article.

Despite their issues, these guides can still be a good option when building a 3D printer on a budget. By discarding the worst rails & carriages and improving the ones with minor issues, a good result can be achieved for a low price.

Genuine Hiwin / Misumi / Japanese linear guides

These are the linear guides at the other end of the spectrum. They are the most expensive, but also of the best quality. They have narrow tolerances, smooth travel and work right out of the box.

Because of their high prices, it is often hard to justify these linear guides for most hobby projects.

It is sometimes possible to find these guides for a decent price on the second-hand market. While they are used, they still are of good quality and have narrow tolerances.

It might be difficult to find these deals however, especially when you want to find the right type of guide rail, the right quantity and the exact lengths you need.

Linear guides from Robotdigg

The Robotdigg linear guide rails, despite also coming from China, are in my experience of better quality than the cheapest Chinese guides. Both the machining and the materials used are of higher quality. They cost a bit more than the cheap Chinese ones, but nothing extravagant.

Out of all options this one seemed like the best value to me, so I used their 440C SUS MGN9 and MGN12 linear guide rails for all axes in my new 3D printer build.

As you can see above, each option comes with its own up- and downsides. In the end it comes down to choosing between spending more money vs. spending more time.

The expensive linear guides cost a lot more, but work perfectly right out of the box. Some of the cheaper guide rails come with problems and defects, which require an input of time and effort to deal with.

In order to deal with the limitations of the Chinese linear guides several things can be done. I have listed some of the common methods to do this below.

Further down the page I have included a guide on how to disassemble and reassemble the carriage blocks.

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Note that in this article I will be using MGN-type linear guide rails as an example. However, the methods listed below also apply to similar types of linear rails, such as the HGR-, MGW- and HGH-series.

Ways in which the performance of cheap linear guides can be improved

Typically when dealing with a sticky carriage, it can be hard to say what the exact cause is. For example, it can be caused by some of the bearing balls being slightly too large, thick grease preventing the bearing balls from rolling freely, or the linear guide rail being slightly oversized for the carriage.

Similarly, the carriage getting stuck at some points along the rail can have various causes as well.

Given that there isn’t one specific solution for each situation, I find it best to try solutions until the problem is solved.

The list of methods below is roughly sorted by amount of effort involved, so I recommend working down from top to bottom.

Removing the stock grease

The cheap Chinese linear rails are often coated in the factory with a thick grease that prevents rust and corrosion. This is useful for when the guides are stored in a warehouse or are being shipped, but not so much when we want to use them in our builds.

The thick grease prevents smooth travel of the carriage on the rail. We can solve this by removing the grease.

A simple way of removing the grease is to thoroughly wipe down the rail and lubricating the carriage with a proper lubricant (see below).

A more complete way of removing the grease is to disassemble the carriage and to degrease all metal components (bearing balls, metal carriage parts and the rail) with a degreaser.

Examples of degreasers that can be used are kerosene (paraffin/lamp oil), mineral spirits (turpentine) or even WD40.

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Make sure not to use any degreaser on the non-metal parts of the carriage. If the degreaser is petroleum based it will eat away at the plastic and/or rubber carriage block components.

Lubricating the linear guide carriage

The steel balls inside the linear rail carriage need to be lubricated with a small amount of appropriate lubricant. The lubricant shouldn’t be too thick. If too much of a high-viscosity lubricant is applied, it will impede the rolling of the steel balls. The lubricant should also not be too thin, because then it will need to be reapplied too often.

Over the years I have tried various lubricants for the linear motion components of my 3D printers, and the one I have settled on is Super Lube 51010 Oil. It is an excellent PTFE-based oil lubricant that lasts for a long time. You can use the Super Lube 51014/51004/51008/51025 product codes as well, as it is the exact same stuff but in a slightly different container or packaging.

There are also some good PTFE-based greases that you can use, but I prefer to avoid those. From what I have noticed, grease attracts more dirt and grime than oil does.

To apply the Super Lube 51010, place a single drop on each of the ball tracks in the carriage, before sliding the carriage on the rail.

The more expensive linear guides have a lubricant application port built into the carriage block, but the cheaper variants do not have this.

Warning icon
Despite regularly being recommended as a lubricant, WD40 should not be used for this purpose. WD40's primary goal is Water Displacement. While it does have some lubricant-like properties, it dries out fast. After WD40 has dried out it starts to collect dust and grime, which is the opposite of what we want.

Running in the carriage

Running in (also known as breaking in) the carriage can be done by moving the carriage back and forth over the rail for a period of time. Ideally this would be done under light load. It allows the moving parts of the carriage block to wear in against each other and create some final size and shape adjustments that allow for smoother travel.

Especially the cheaper linear rails and carriages can benefit from this, as they often come with minor imperfections that can be evened out using this method.

It is hard for me to say what would be the best amount of time to run in the carriage, but I have had success with moving the carriage back and forth for about 15-30 minutes. In the end it will depend on the severity of the imperfections. That said, this is not a method that can fix absolutely everything.

Replacing the stainless steel bearing balls

Some of the cheap linear guide rails come with inferior quality stainless steel bearing balls. For example, some of the bearing balls can be non-spherical or have a diameter that is a bit too far off from optimal. Replacing the bearing balls with better quality ones can improve performance and give smoother travel.

You can also find that the carriage has some bearing balls missing, however, this is not always that big of a deal. When one or more balls (up to a certain point) are missing, the remaining ones simply have to take on a slightly bigger load.

In 3D printers (as opposed to CNC machines) the loads are typically not very high, and thus there is little risk of exceeding the load ratings of the carriage.

The easiest way to replace the bearing balls is to open up the carriage block (see further down the page), take out the balls and reassemble the carriage block with the new higher quality bearing balls.

The size of the bearing balls in the carriage depends on the size of the linear rail:

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The more expensive linear rails usually have their carriages exactly matched to the rail to get the best fit. Replacing the bearing balls of those carriages will likely have a non-desired effect. Then again, these rails are unlikely to have any of the issues that would require replacing the bearing balls in the first place.

Reaming the carriage channels

The carriage block contains two channels through which the stainless steel balls are circulated. It can happen that during manufacturing these channels are not reamed or deburred properly. This can cause the surface of the channels to have a rough surface. The rough surface causes the stainless steel balls to travel less smooth, or to even get stuck periodically.

To solve this you can place the carriage block in a drill press and use an appropriately sized drill bit to smooth out the channels.

The channels are unlikely to end up perfectly smooth, but depending on the original state of the channels there might be a significant improvement.

A close-up of one of the ball channels on the inside of a disassembled MGN12H linear rail carriage block.
If the inside of the ball channels has a rough finish, the bearing balls might not be able to circulate properly.
A metal MGN12H carriage block component with PTFE inserts.
The channels in the Robotdigg linear rail carriages have a smooth PTFE(?) insert. I haven’t seen them in the other Chinese linear guides, so this might be part of why the Robotdigg linear rails perform a bit better.

Here is a summary of the fixing methods

  • Removing the stock grease from the rail and carriage.
  • Lubricating the carriage with a proper lubricant.
  • Running in the carriage.
  • Replacing the stainless steel bearing balls.
  • Reaming the carriage channels.

Read on to learn how to disassemble the carriage blocks for cleaning.

Materials

Tools

  • Small Phillips-head screwdriver
  • Small flat-head screwdriver

Optional but useful

  • Clear transparent bag

How to disassemble MGN carriage blocks

A MGN12H carriage being removed from a MGN12 linear rail inside of a transparent bag.
Remove the carriage block from the linear rail. In order to disassemble the carriage block, we first need to remove it from the rail. I recommend doing the removal process in something like a clear bag or container, because the small bearing balls can jump away sometimes.
To make things easier for myself I have omitted the bag from the rest of the pictures.
A screwdriver being used to remove one of the screws of a MGN12H carriage.
Unscrew the screws on the side of the carriage. These screws hold the entire carriage assembly together, so we need to remove them.
Warning icon
Try not to use any magnetized tools. If the metal parts of the carriage block end up magnetized, the bearing balls will not circulate as smoothly.
A MGN12H carriage placed on a desk surface with next to it one of its rubber seals.
Take the rubber seal off after the screws have been unscrewed. Repeat this and the last step for the other side of the carriage.
A hand using a small flathead screwdriver to remove one of the retaining wires on a MGN12H carriage.
Un-clip the two metal retaining wires from the assembly. To do this use a small flat-head screwdriver to gently nudge the retaining wire out from the outside of the carriage.
Be careful during this step, the retaining wire needs to be slightly bent in order to remove it, but we do not want to bend it so much that it permanently deforms.
This is also the step where all remaining bearing balls will come out, so be careful with that as well. They are hard to find once they have escaped and found their way to freedom.
A fully disassembled MGN12H linear rail carriage with all components laid out on a surface.
Perform the improvement methods on the carriage block components. Now that the carriage block is fully disassembled, you can clean and degrease the components, replace the bearing balls and/or drill out the ball channels.

A summary of the disassembling process

  1. Remove the carriage block from the linear rail.
  2. Unscrew the screws on the carriage block.
  3. Remove rubber seals.
  4. Un-clip the metal retaining wires.
  5. Don’t lose the stainless steel ball bearings.

Reassembling the carriage block

Reassembling the carriage is essentially not much different from disassembling it, except with the above steps in reverse.

The bearing balls get inserted manually one by one at the end, after the carriage has been fully assembled. This process is a bit finicky, but patience will get you there.

There might be a better and faster way to insert the bearing balls, but I haven’t found it yet. If you know of a way, please let me know in the comments.

Conclusion

With the methods above I have had good success at fixing the issues that usually come with cheaper linear guide rails. I definitely haven’t had a 100% success rate, but so far I have found it worth my time to give most of these methods a shot (with the exception of reaming out the carriage channels).

Interestingly most of the issues I have had have been with MGN12 rails. Nearly all of the cheaper MGN9 rails that I have purchased worked flawlessly right from the start.

I am not sure if there is a difference in manufacturing between the sizes, or I have just been unlucky with the larger rails, but either way I find it an interesting pattern.

Last update on 2020-08-08 / Affiliate links / Images from Amazon Product Advertising API / Affiliate Disclosure

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